Do Cardiac Actin Mutations Lead to Altered Muscle Contractility?

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The hypothesis that increased heart muscle contractility leads to HCM and reduced contractility leads to DCM is based mainly on studies on myosin motor proteins. This hypothesis predicts that a change in cardiac actin (ACTC) may impact muscle contractility. To test this prediction, we determined the interactions between myosin and 8 of 16 ACTC mutant proteins that are known to cause hypertrophic or dilated cardiomyopathy. R312H showed a decreased actin-activated S1 ATPase rate (13.1±0.63 μM/min) compared to WT (15.3±1.58 μM/min), whereas the rate with E99K was significantly higher (20.1±1.46μM/min). In vitro motility assays were performed with varying ATP concentrations. E99K exhibited increased KM (0.086±0.02 μM) compared to WT (0.068±0.02 μM). E99K demonstrated a significantly decreased Vmax (1.93±0.1 μm/sec) compared to WT (3.31±0.12 μm/sec). In contrast, M305 had a similar KM (0.088±0.01 μm/sec) to E99K, but its Vmax (3.39±0.11μm/sec) was similar to WT. Based on a 5 nm myosin step size, we calculated a duty ratio of about 0.04 for WT and most mutant actins; however, the duty ratio was twice as high for E99K. With thin-filament extracted and reconstituted muscle fibers with E99K, a slight, but significant increase in Ca2+sensitivity (ΔpCa=0.11±0.04) was observed; consequently tension was slightly larger than that of WT for pCa ranging 5.6-6.0, without changing maximum tension at pCa 4.66. These experiments suggest that the primary change with the E99K actin mutant is enhanced ATP usage and increased tension at partial activation, which lead to HCM.